The present invention relates to a system for generating or sensing transverse elastic bulk waves in solids. One of the principal uses of bulk waves in solids is the detection and isolation of fractures or other faults in the solid material, as in non-destructive testing of materials. Such testing is of prime importance in the field of atomic energy, and even more particularly, in the construction and operational testing of nuclear reactors.
In non-destructive testing, an elastic bulk wave is transmitted through the solid being tested, and the transmission characteristics are monitored either at a remote location or upon reflection of the wave. Various signal processing techniques are then used on the sensed signal to determine whether the bulk wave has passed through a faultless solid or, alternatively, whether there exists a fault somewhere in the solid.
A transverse bulk wave in a solid material is characterized by an oscillatory motion of particles in the solid perpendicular to the direction of propagation of the wave (i.e., in the shear plane) within the material; and this characteristic is due to the inherent elastic nature, within limits, of solid matter. Thus, the waves are sometimes referred to as bulk shear waves.
As will be explained in more detail within, heretofore bulk waves have been generated by means of a piezoelectric transducer having a sandwich-type construction with the piezoelectric material in the center and conductive electrode pads fixed to opposite surfaces of the substrate. Bulk waves are generated within the piezoelectric substrate by applying an alternating voltage to the two surface electrodes. These bulk waves are then coupled from the substrate through one of the surface electrodes into the solid material being tested. Similarly, bulk waves are sensed by bonding one of the electrodes to the material under test to transmit bulk waves from the material into the piezoelectric substrate. The bulk waves in the piezoelectric substrate are then sensed at the electrodes.
Piezoelectric materials have long been known in the art, and they are characterized in that if an electric field is applied to the material, it will tend to distort; and conversely, if a stress is applied to the material, an electric field will be generated. In piezoelectric materials used for bulk shear wave generation when an electric field is applied, the material distorts in a plane perpendicular to the field vector.
The present invention makes use of a piezoelectric material which is characterized in that it has a two-fold axis or equivalent symmetry--that is, it includes any combination of symmetry elements in which a two-fold axis of symmetry exists. A pair of electrodes are formed on a surface of the substrate which is parallel to the two-fold axis; and each electrode preferably takes the form of a plurality of elongated, interconnected conductive fingers which are also parallel to the two-fold axis. The conductive fingers of each electrode are interlaced to form a pair of interdigital electrodes.
The substrate is preferably bonded to the material being tested along one side which is parallel to the two-fold axis and perpendicular to the surface on which the interdigital electrodes are formed. When used as a generator of bulk shear waves, an alternating voltage source is connected to the electrodes to apply an alternating electric field in the material transverse of the two-fold axis; and this generates surface waves which propagate along the surface on which the electrodes are formed and perpendicular to the two-fold axis of the crystal. Normally, surface waves excited in a solid decay very rapidly with depth, and they can be said to be truly confined to the surface. However, with the piezoelectric material having a two-fold axis, we have found the decay of surface waves with depth may be very much less such that they are almost indistinguishable from bulk waves, as will be more fully explained below. We call such waves as are generated according to our invention deep surface waves because of the relatively small attenuation of the surface waves. Without so limiting our invention, we postulate that it is this characteristic which facilitates transformation of surface waves on the transducer to and from bulk waves in a solid. These surface waves are thus transformed to the material under test as bulk shear waves. Whether used as a sensor or generator of bulk shear waves, the orientation of the transducer relative to the material being tested and the formation of electrodes on the transducer is the same. That is, bulk waves in the material tested are coupled out of a surface perpendicular to the surface of the transducer on which the electrodes are formed and parallel to the two-fold axis. Corresponding waves in the transducer then form deep surface waves which may be sensed by the interdigital electrode configuration.
The transducer of the present invention, by eliminating the metallic electrode interposed between the solid material under test and the piezoelectric material, as required in prior devices, yields a system which is more sensitive and more efficient than prior transducers because the interposed metallic surface of prior devices is usually acoustically different than either the piezoelectric material or the solid material under test, and it therefore produces reflections at the interfaces. Further, as will be explained in greater detail below, the present device is capable of operating at a much higher frequency of operation than are the prior devices. This is due principally to the fact that the operating frequency of the inventive transducer is dependent upon the design of the interdigital electrodes, but is independent of the thickness of the piezoelectric material. That is, the operating frequency of prior devices is determined by the thickness of the material between the two electrodes; whereas the operating frequency of the present device is independent of the thickness of the piezoelectric substrate.
Further, the present system affords greater operational flexibility because the bandwidth can be varied by electrode design, as will be explained more completely below. Another advantage of the present invention is that the fabrication of the transducer is greatly simplified, using well-developed techniques in deposition of metals to form the interdigital electrode, and the device is simply bonded or placed in intimate engaging contact with the surface of the material being tested with a suitable adhesive or fixture. The device of the present invention may be used in all systems wherein elastic bulk shear waves are generated or monitored.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.